To provide a small cover, while coping with replacement of long tools.

This cover 1 is openable/closable, and covers a workpiece region W arranged with a main shaft R in a closed state. The cover 1 is provided with: a vertically directed first cover 2 turning around a vertical side from the closed state to an opened state; a vertical second cover 4 adjacent to be continuous to the first cover 2 of the closed state in the closed state and adjacent to cover the first cover 2 of the opened state in the opened state; and an arm 22 for making the second cover 4 differentially rotate to perform rotary moving up to each adjacent position of the first cover 2, while performing rotation of the second cover 4 itself, from the closed state to the opened state.